As the size of BGA or LGA packages is reduced, the space occupied by the terminals is also reduced, making it necessary to decrease the pitch of the terminals. In conformity with this, contacts that are mounted in the sockets are arranged corresponding to the pitch of the terminals. U.S. Pat. No. 6,749,443 relates to a socket for mounting a semiconductor device of the surface mounting type such as BGA and CSP devices. A socket capable of adjusting the amount of deformation of the terminals is provided pursuant to this patent by providing contact restraining members for limiting the position of the end of the contacts.
Japanese Patent Application No. 2004-152554 shows a socket in which potential dislocation of the BGA package is prevented when the terminals are pulled away from the end of the contacts by positioning and holding the BGA package that has been placed in the direction of the diagonal line by means of a positioning mechanism that is linked to the movement of the cover member.
When the pitch of the terminals of a BGA or LGA package becomes narrower, the contacts are arranged at a pitch corresponding thereto, with a consequence that the contact area between each terminal and the respective contact becomes smaller and this develops a tendency for contact resistance to become greater. In the case where the terminals are solder balls of a bump shape, as in the case of a BGA package, the shape of the tip of the contacts is made like a V or a U as shown in FIG. 9 for the purpose of preventing any deformation of the lowermost surface of the solder balls, with a result that the contact resistance becomes greater.
An increase in contact resistance greatly affects the measurement accuracy when the semiconductor device is tested, leading to an increase in errors. This problem can be dealt with by using a Kelvin technique in which separate contacts are used for the supply of electric power and for the measurement of the signal. Accordingly, using this technique, the number of contacts required is doubled. However, doubling the contacts of a prior art structure as shown in FIG. 9, is not feasible for use with such narrow-pitch semiconductor packages. Further, if the number of contacts is doubled and the pitch of the contacts is concomitantly narrowed, it becomes difficult to form through-holes in the substrate for connection of the contacts that protrude from the socket.